That big blue field is a whole bunch of cachegrind.out files in /private/var/tmp. Turns out these files are generated when you have Xdebug enabled, which I do because it's useful to debug PHP as a developer. So, first thing I try to do is delete all these useless files.
I changed where Xdebug saves it's cache files to /tmp in my php.ini file. In my case with MacPorts it's at /opt/local/etc/php54/php.ini.